Colin Thornton

Loess soils undergo collapse when the material is loaded and wetted, which leads to engineering problems and disasters. To understand better the collapse mechanisms that govern loess and loess-like deposits under complex stress paths, a distinct element model has been established. A contact model considering water content effect introduced for structural loess was used to cement contacted particles together to simulate unsaturated structural loess under biaxial compression conditions. The numerical structural loess was tested for different wetting and loading paths, i.e., loading-quick wetting (LW^Q), loading-gradual wetting (LW^G), and wetting-loading (WL) paths. The simulated macroscopic mechanical behavior agrees approximately quantitatively in terms of the deviatoric deformation and qualitatively regarding the volumetric deformation when compared with available experimental results. The tests under different mean stresses indicate the existence of initial, maximum, and terminating collapse stress states in a biaxial wetting test. The influence of the wetting procedure, i.e., gradual wetting or quick wetting, has a considerable effect on the mechanical performance for numerical samples. The axial strain of a sample during a loading-wetting (LW) path is larger than that of a sample in a WL path. The wetting-induced volumetric deformation corresponds with the failure of bonded contacts. The deviator fabrics (fabric anisotropy) of total, bonded, frictional, and broken bond contacts were also investigated.

The paper presents results of three-dimensional discrete element method (DEM) simulations of axisymmetric undrained tests on loose assemblies of polydisperse spheres using a periodic cell. In the work reported, undrained tests were modeled by deforming the samples under constant volume conditions. The undrained (effective) stress paths are shown to be qualitatively similar to published physical experimental results. The onset of liquefaction (or temporary liquefaction) is identified by a redundancy index equal to unity, which defines the transition from solidlike to liquidlike behavior. This corresponds to a critical mechanical coordination number slightly in excess of 4. The results of the simulations also suggest that a reversal in the direction of the undrained stress path does not necessarily indicate temporary liquefaction. The undrained behavior obtained by the DEM simulations is found to be dependent on strain rate and the so-called temporary liquefaction phenomenon is only observed if the ...

Strain localisation and shear band formation is frequently observed during the handling and flow of dense phase particulate materials. However, a complete understanding of how shear bands form and what happens inside shear bands is still lacking. In order to address this problem, discrete particle simulations have been carried out to examine the detailed processes that occur at the grain scale associated with the initiation and development of shear bands. To reliably identify the continuum model applicable within a shear band is difficult due to the small number of particles/contacts involved. However, it is normally accepted that the mode of deformation within a shear band is one of simple shear. Consequently, simple shear simulations have been performed in order to determine the evolution of the stress tensor, dilation rate, and the principal directions of stress and strain-rate. It is demonstrated that the corresponding non-coaxial flow rule is equivalent to that suggested by Tatsuoka et al. (Géotechnique 38 148 (1988)). Furthermore, at fully developed flow when there is no further change in volume, the stress and strain-rate directions are coaxial and the flow rule is that proposed by Hill (The Mathematical Theory of Plasticity (Oxford University Press, 1950) p. 294).